Molecular receptors such as boronates can be used in sensors for the detection and/or measurement of analyte in biological fluids. For example, a sensor may comprise a glucose receptor (the boronic acid) and a fluorophore that acts as the transmitter of the signal. These indicator chemistries can readily be immobilised onto an optical fibre of appropriate diameter, which can then be placed into body fluids or tissue to measure analytes such as glucose.